Introduce a feature-contributor role in Splice

[isegall-da]

There are organizations and individuals that contribute actively, who are working on complex features that span more than one PR. They are:

• trusted in the sense of CI costs
• trusted with "Splice secrets", but not "DA secrets"
• trusted in terms of code on the feature branch, so can create PRs, respond to approve-with-feedback reviews, and merge to the feature branch. They can also approve and merge PRs to a feature branch, contributed by others.
• not yet ready to approve merges to main

In the current state, too much burden is placed on the "full-contributor" to maintain the feature branch. We want to officially introduce the notion of Feature Contributors, with the capabilities above, and introduce tooling to support that.

[isegall-da]

We're parking this for now, as it doesn't seem to be easy to support in the current setting. Once we can create more forks of Splice easily, we will revisit this using feature-forks, which can have different permissions, but still access to the CI runners.